Ascorbic acid (vitamin C) has functions such as inhibition of lipid peroxide production, inhibition of collagen formation, retardation of melanin formation and enhancement of immune functions, and has heretofore been used in the fields of medicines, agricultural chemicals, animal drugs, foods, feeds, cosmetics, and the like for these purposes.
However, since ascorbic acid is poor in stability and has also poor lipid solubility, the amount thereof which permeates through the cell membrane and accumulates in the cells is also limited. For this reason, there is a trend in that by the use of ascorbic acid itself, the physiological actions of vitamin C cannot necessarily be achieved to a sufficient degree.
In order to improve the stability and lipid solubility of ascorbic acid, various ascorbic acid derivatives have been proposed. It has been thought that among the ascorbic acid derivatives, the ascorbic acid-2-phosphoric acid derivatives in which the hydroxyl group at the 2-position of ascorbic acid is esterified with a phosphoric acid are difficult to oxidize in air, and thus exhibit excellent stability. In addition, it has been considered that the derivatives in which the hydroxyl group of ascorbic acid is acylated with a higher fatty acid is excellent in lipid solubility.
In Patent Document 1, a method of producing a 6-O-higher acylascorbic acid-2-phosphoric acid ester and results of the structure determination of the substance have been described. It has been described that the 6-O-higher acylascorbic acid-2-phosphoric acid ester produced by this method is improved both in stability and in lipid solubility, and is easily taken up into cells. The 6-O-higher acylascorbic acid-2-phosphoric acid esters are usually used in the form of salts.
An emulsified skin external preparation containing 0.03 to 25% by mass of a salt of a 6-O-higher acylascorbic acid-2-phosphoric acid ester and 0.05 to 25% by mass of a glycerin monofatty acid ester has been disclosed in Patent Document 2. It has been described that in the emulsified skin external preparation, the decrease of the salt of 6-O-higher acylascorbic acid-2-phosphoric acid ester due to decomposition can be suppressed by the coexistence of the glycerin monofatty acid ester.